This invention relates to a signal transmission cable comprising a conductor portion for transmission of an electrical signal and an insulator sheath covering the conductor portion and, in particular, to such a signal transmission cable having a noise absorber suppressing noise leaking out of and invading into the cable.
In order to transmit electrical signals such as communication signals between electronic devices and between electronic apparatus, use is made of signal transmission cables such as communication cables. A typical one of the transmission cables usually comprises a conductor portion for transmission of signals therethrough and an outer insulator sheath surrounding the conductor portion. A coaxial type of the signal transmission cables comprises a central conductor portion for transmission of signals therethrough, an outer conductor portion to be grounded, an insulator layer interposed and insulating between the central conductor portion and the outer conductor portion, and an outer insulator sheath surrounding the outer conductor portion. It is well known as the so called electromagnetic interference (EMI) that high frequency electrical noise is generated from active electronic elements, high frequency circuit components, and high frequency electronic apparatus, flows through the signal transmission cable and is radiated from the cable. On the contrary, electrical noise invades through the signal transmission cable to those active electronic elements, high frequency circuit components, and high frequency electronic apparatus.
It is well known in the art that a cylindrical ferrite core is attached onto an electric power code to an electronic apparatus, for example, computer so as to suppress a high frequency noise from flowing into, or from, the computer through the electric power code. The ferrite core absorbs the high frequency noise current flowing through the power code. The ferrite core used has a large volume in comparison with electronic apparatus which have rapidly been small-sized with electronic circuit components disposed at a high density.
It is also well known in the art that a concentrated constant circuit such as a decoupling capacitor is assembled in a power circuit line in the electronic apparatus so as to suppress undesired radiation from the power line.
It is also another problem that a high frequency noise is often caused or induced from a semiconductor or an integrated circuit device of a high speed operation type such as a random access memory (RAM), a read only memory (ROM), a microprocessor (MPU), a central processing unit (CPU), or an image processor arithmetic logic unit (IPALU) because an electric signal flows in a high speed circuit therein with rapid change in current and voltage value.
In addition, electronic elements and cables are disposed with a high density in a small-sized electronic apparatus. Therefore, those elements and lines are very close to each other and thereby affected to each other to cause EMI.
In order to suppress the high frequency noise from those semiconductor devices and the EMI within the small-sized electronic apparatus, the conventional ferrite core cannot be used because it has a relatively large volume.
On the other hand, use of the concentrated constant circuit cannot sufficiently suppress the high frequency noise caused in the circuit using electronic elements of the high speed operation type because the noise has an increased frequency so that the circuit line actually acts as a distributed constant circuit.
Japanese Unexamined Patent Publication (JP-A) H11-185542 discloses a cable with a thin-film magnetic shield. The cable is generally used as an interface cable for connecting OA (office automation) apparatus such as a personal computer, game apparatus, and communication equipment to one another and as an internal wiring cable for connection of various components in the apparatus.
A first conventional cable with a thin-film magnetic shield is disclosed in the above-mentioned Japanese publication and comprises a plurality of signal conductors as a conductor portion arranged at the center for transmission of signals, an insulating tape wrapped around the conductor portion, a laminated tape wrapped around the insulating tape, and an insulator covering the laminated tape. The laminated tape comprises a laminate of a metal leaf or foil having high conductivity and at least one high-permeability thin film made of a material having high permeability.
With this structure, radiation noise is effectively shielded. Specifically, since the metal leaf (typically, copper leaf) having high conductivity is surrounded by the high-permeability thin film, the radiation noise surviving through the metal leaf without being absorbed thereby can be shielded by the high-permeability thin film. Thus, the radiation noise is first shielded by the metal leaf, and then by the high-permeability thin film arranged therearound. As a consequence, the above-mentioned cable is improved in shielding effect over a wide range, easy in handling, and smart in appearance because the diameter of the cable need not substantially be increased.
A second conventional cable with a thin-film magnetic shield is also disclosed in the above-mentioned publication. This cable is similar in structure to the first conventional cable mentioned above except that the insulating tape is provided with slits. With this structure, the cable as a whole is prevented from occurrence of an antenna effect and the influence of eddy current of the high-permeability thin film is suppressed. Therefore, it is possible to suppress the radiation noise over a wide frequency band.
However, the high frequency current or the high frequency radiation noise contains a harmonic component. In this event, a signal path exhibits the behavior as a distributed constant circuit. Therefore, the conventional countermeasure against the noise is not effective because such countermeasure assumes the lumped constant circuit.
In the above-mentioned publication, the high-permeability thin film is typically a magnetic thin film formed by rolling a permalloy (Fexe2x80x94Ni alloy). Such magnetic thin film as the high-permeability thin film has following problems. Specifically, the frequency characteristic (xe2x80x9cfxe2x80x9d characteristic) of the magnetic characteristic thereof is inferior particularly at the high frequency. In addition, the electric characteristic is degraded.
Alternatively, use may be made of the high-permeability thin film made of a Co-based amorphous material, for example, Coxe2x80x94Fe alloy. In this case, however, the frequency characteristic of the magnetic characteristic thereof is inferior particularly at the high frequency, like in the above-mentioned case. Furthermore, although the Co-based amorphous material can be manufactured in a laboratory, the cost is high. Accordingly, this material can not practically be used in the industry.
It is therefore an object of this invention to provide a signal transmission cable capable of efficiently suppressing only a high frequency noise.
It is another object of this invention to provide a signal transmission cable capable of achieving the above-mentioned effect without requiring any additional space.
This invention is applicable to a signal transmission cable comprising a conductor portion for transmitting an electric signal therethrough and an insulator sheath covering said conductor portion. A typical example of the signal transmission cable is a coaxial cable further comprising an outer conductor portion around said conductor portion and an inner insulator layer disposed between said conductor portion and said outer conductor portion, said outer conductor portion being directly covered with said insulating sheath. According to this invention, the signal transmission cable is provided with a high loss magnetic film formed on at least one area of said insulator sheath and covering at least a part of an outer surface of said sheath. The high loss magnetic film has the maximum complex permeability xcexcxe2x80x3max in a frequency range of 0.1-10 gigahertz (GHz).
It is preferable that the high loss magnetic film has a DC specific resistance of 100 xcexcxcexa9xc2x7cm or more.
It is also preferable that the high loss magnetic film has a thickness of 0.3-20 xcexcm.
According to an embodiment, the high loss magnetic film is a thin film formed by sputtering process, or alternatively by vapor deposition process.
It is preferable that the high loss magnetic film is covered with an outer insulating sheath.
The high loss magnetic film is preferably made of a M-X-Y magnetic composition which is comprising M, X and Y, where M is a metallic magnetic material consisting of Fe, Co, and/or Ni, X being element or elements other than M and Y, and Y being F, N, and/or O, said M-X-Y magnetic composition having a concentration of M in the composition so that said M-X-Y magnetic composition has a saturation magnetization of 35-80% of that of the metallic bulk of magnetic material comprising M alone.
According to an embodiment of this invention, the M-X-Y magnetic composition has a saturation magnetization which is 60-80% of the saturation magnetization of the metallic magnetic material M alone. The M-X-Y magnetic composition has a complex permeability frequency response of a relatively narrow band where a relative bandwidth bwr is 200% or less. The relative bandwidth bwr is determined as a percentage ratio of bandwidth between two frequency points which shows the complex permeability as a half value xcexcxe2x80x350 of the maximum xcexcxe2x80x3max, to center frequency of said bandwidth. The M-X-Y magnetic composition has a DC specific resistance of 100-700 xcexcxcexa9xc2x7cm.
According to another embodiment, the M-X-Y magnetic composition has a saturation magnetization which is 35-60% of the saturation magnetization of the metallic magnetic material M alone. The M-X-Y magnetic composition has a complex permeability frequency response of a relatively broad band where a relative bandwidth bwr is 150% or more. The relative bandwidth bwr is determined as a percentage ratio of bandwidth between two frequency points which shows the complex permeability as a half value xcexcxe2x80x350 of the maximum xcexcxe2x80x3max, to center frequency of said bandwidth. The M-X-Y magnetic composition has a DC specific resistance of 500 xcexcxcexa9xc2x7cm or more.
The M-X-Y magnetic composition is a granular magnetic composition wherein said metallic magnetic material M is distributed as granular grains in a matrix composition consisting of X and Y The granular grains preferably have an average grain size of 1-40 nm.
Typically, X is at least one selected form a group consisting of C, Bi, Si, Al, Mg, Ti, Zn, Hf, Sr, Nb, Ta, and rare-earth metals.
According to an embodiment, the M-X-Y magnetic composition is a composition represented by a formula of Fexcex1xe2x80x94Alxcex2xe2x80x94Oxcex3.